1. Field of the Invention
The present invention relates to a well jarring tool used to free stuck pipe within a well bore. More particularly, the present invention relates to a double acting hydraulic well jar useful in coiled tubing and conventional drilling applications which includes a metering system to establish a predetermined timing sequence prior to applying either an upward or downward jarring action.
2. Description of the Related Art
Jarring tools are used to free stuck drill pipe or well tools in a well bore. They provide a substantial upward or downward jarring action in an effort to transmit sufficient force to dislodge a stuck member. Double acting jars which can transmit either upward or downward jarring loads are well known in the prior art. See, for example, U.S. Pat. Nos. 4,186,807; 4,865,125; and 5,007,479. Such jars typically use a hydraulic-type fluid to isolate well bore pressure and provide the working fluid through which the jarring tool operates.
With the advent of coiled tubing techniques, the need exists for a variety of downhole tools capable of performing their traditional functions but in the confines of a coiled tubing application. Briefly, a coiled tubing operation involves the use of a single continuous pipe or tubing for drilling application rather than the more traditional 30-foot drill pipe sections. The tubing, which is coiled onto a reel and uncoiled as it is lowered into the well bore, can be used for either drilling or workover applications. However, coiled tubing presents a number of working constraints to existing tool design. First of all, due to the limited size of the coiled tubing, limited compressive loads can be placed on the tubing by the rig operator. Essentially, this means that downhole tools which require compressive force to operate, such as a jarring tool, must be capable of operating with the limited compressive load capability of coiled tubing. In addition, in coiled tubing application the overall length of the downhole tool becomes significant since there is limited distance available between the stuffing box and the blowout preventor to accommodate the bottom hole assembly. A typically bottom hole assembly might include a quick disconnect, a sinker bar located below the quick disconnect to provide weight to the bottom hole assembly, the jarring tool, a release tool below that of some type, and then an overshot. There may be other tools as well which may be needed. Thus, the length of the jarring tool itself becomes particularly significant since the entire bottom hole assembly must fit within the limited distance between the stuffing box and blowout preventor to introduce it into a pressurized well. Furthermore, within these confines, the jarring tool must have a large enough internal bore to permit pump-down tools to pass. Thus, the coiled-tubing jarring tool must have a limited overall wall thickness in view of limited outer diameter conditions.
As in the case of traditional drill pipe, coiled tubing or other down hole tools may get stuck in the well bore at times. Under these circumstances, repetitive upjarring or downjarring with a jarring tool is particularly advantageous. Thus, the need exists for a double acting hydraulic jarring tool which can satisfy the limited compressive load, limited length and large bore requirements of coiled tubing application as mentioned above but which also employs a metering principle to provide sufficient time for the operator to prepare for an upjarring or downjarring activity and provide for multiple jarring applications. Preferably, such a jarring tool design would have application in a conventional drill string as well.